The invention pertains to an organic light emitting diode (LED) comprising a transparent electrode, superposed by a layer of a conductive transparent polymer (CTP), superposed by a layer of a light emitting polymer, oligomer, or low molecular weight compound, superposed by a metal cathode, wherein the CTP layer has a low sulfate and high metal ion content.
LEDs comprising a conductive transparent polymer are known in the art to increase the lifetime of polymer LEDs essentially. LEDs with CTP layers have been described, for instance, in patent application WO 96/08047. Various polymeric materials have been described as suitable for use in LEDs as materials that on the one hand provide areas of electric insulation, and at the other hand provide electroconductive areas that serve as real hole-injection anodes. Such polymers that, depending on the location, can be in the conductive and non-conductive state, have been found in the classes of polythiophenes, polyamines, polypyrroles, polyanilines, and polyacetylenes, which polymers may be substituted with alkyl and alkoxy groups, halogens, and the like. The thickness of these CTP layers is reported to be between 50 and 500 nm. Thin layers are preferred to prevent substantial absorption of the visible light, which decreases the performance of the LED. However, in practice these layers must be relatively thick to prevent leakage currents, short-circuit, and pinholes, and to improve the device lifetime.
The major disadvantage of these CTP layers is the complicated and expensive method of making layers with conductive and non-conductive areas, e.g. for use in a matrix or segmented organic LED displays. It would be a substantial advantage to make layers that have the same conductivity over the whole layer. Pixels that are electrically connected through the CTP layer suffer from an inhomogeneous decay of emitted light within one pixel under multiplexed driving, the so called anode shrinkage. Examples of multiplexed driving can be found in U.S. Pat. No. 6,014,119. Typically, anode shrinkage is first visible at the sides of the display and propagates row after row, or column after column inside the matrix display. In general anode shrinkage can appear between any two pixels or segments of a display. Moreover, generally layers of these CTP polymers influence the lifetime of the device due to compounds included in the dispersion, solution, or suspension. It is an object of the present invention to improve the pixel degradation due to the shrinkage and to improve the lifetime of the device. Also by applying ink jetting for the deposition of the CTP layer, it is possible that a connected CTP layer covering more than one pixel is easier and cheaper to realize than preparing a substrate architecture with separation walls around one pixel that prevents the CTP dispersion, suspension, or solution, dispensed to one pixel, to cover other pixels also. Such pixels, which are electrically connected via the CTP layer suffer from shrinkage under multipled driving.
It has now been found that when the CTP layer has a sulfate ion (SO42xe2x88x92) content less than 7,500 ppm, and a metal ion content more than 0.04 mmoles/g, the lifetime and the anode shrinkage of the LEDs is considerably improved. The present invention therefore pertains to an organic light emitting diode (LED) comprising a transparent electrode, which preferably is a layer of indium oxide or indium-tin oxide (ITO), superposed by a layer of a conductive transparent polymer (CTP), superposed by a layer of a light emitting polymer, oligomer, or low molecular weight compound, superposed by a metal cathode, characterized in that the CTP layer has a sulfate ion content of less than 7,500 ppm, and a metal ion content of more than 0.04 mmoles/g.
The conductive transparent polymer can be made to a layer by any suitable method, and in general is deposited by a spin coating process from a solution, dispersion, or suspension, or by an ink jet process. For application in an organic LED display, the dried film of the conductive transparent polymer preferably satisfies one or more of the following properties to achieve a long lifetime and a minimum anode shrinkage, as well as a large efficiency:
The conductive transparent polymer is selected from of the class of polythiophenes, polypyrroles, polyanilines, polyamines, and polyacetylenes.
The salt or polyelectrolyte serving as a counter ion is selected from a sulfonic acid, such as polystyrenesulfonic acid and para-toluenesulfonic acid.
The sulfate concentration in the dried film is less than 1,250 ppm.
The metal ion concentration in the dried film is more than 0.5 mmoles/g to avoid anode shrinkage in a matrix or segmented display.
The metal ion is preferably an alkali or earth alkali metal ion. With more preference the metal ion is substantially an ion of K, Rb, Cs, Mg, Ba, and/or Ca.
The preferred requirement that the metal ions are substantially only K, Rb, Cs, Mg, Ba, and/or Ca is of particular importance when the CTP layer is used on an active matrix substrate to realize a long lifetime of the device and to avoid anode shrinkage. The metal ion concentration has then to be high, but the Li and Na ion concentration should be as low as possible, preferably less than 1010 ions/cm2, to avoid MOS instability effects and to optimize the gate dielectrics. The metal ions can be added in the form of their oxides, hydroxides, or salts. Suitable salts are, for instance, carbonates, nitrates, halides (fluorides, chlorides, bromides, and iodides), organic salts, and mixtures thereof.
The sulfate content in the dried layer must be less than 7,500 ppm, which is obtained when the sulfate content is less than 300 ppm in a 4 wt. % solid CTP dispersion. A sulfate content in the dried layer of 1,250 ppm corresponds to a content of 50 ppm in a 4 wt. % dispersion that is used for making the CTP layer.
The metal ion content in the dried layer must be more than 0.04 mmoles/g, which is obtained when the metal ion content is more than 1.6 xcexcmol/g in a 4 wt. % solid CTP dispersion that is used for making the CTP layer. An metal ion content in the dried layer of 0.5 mmoles/g corresponds to a content of 0.02 mmoles/g in a 4 wt. % dispersion.
The polymers that are suitable for the CTP layer are known in the art. Particularly, reference is made to patent application WO 96/08047, which discloses various materials and methods of preparation thereof, and which contents are incorporated by reference. Useful polymers are polythiophenes, polypyrroles, polyanilines, polyamines, and polyacetylenes. Particularly useful are poly-3,4-ethylene dioxythiophene, polyaniline (PANI), and polyurethanes such as ConQuest (ex. DSM, The Netherlands). The polymer may be mixed with monomers, oligomers, or other polymers. A preferred material for use in the CTP layer is the mixture of poly-3,4-ethylene dioxythiophene and polystyrene sulfonic acid (PEDOT).
The active layer is situated between two electrode layers of electroconductive materials. At least one of said electrode layers must be transparent or translucent to the emitted light in the active layer. One of the electrode layers serves as the (positive) electrode for injecting holes into the active layer. The material of this electrode layer has a high work function and is generally formed by a layer of indium oxide or indium-tin oxide (ITO). In addition, such layers are transparent to the emitted light in the active layer. Particularly ITO is suitable because of its satisfactory electrical conductivity and high transparency. The other electrode layer serves as the (negative) electrode for injecting electrons into the active layer. The material for this layer has a lower work function and is generally formed from a layer of, for example, indium, calcium, barium, or magnesium.
The electrode layer of ITO is provided by vacuum evaporation, sputtering, or a CVD process. This electrode layer and often also the negative electrode layer, for example, of calcium, are structured in accordance with a pattern by means of a customary photolithographic process or by partly covering it with a mask during the vacuum deposition process which corresponds to the desired pattern for a display. In a typical example of a display, the electrodes of the first and second electrode layers have line structures which intersect each other at right angles and hence form a matrix of separately drivable rectangular LEDs. The rectangular LEDs constitute the pixels or picture elements of the display. If the electrodes of the first and second electrode layers are connected to an electrical source, light-emitting pixels are formed at the intersection of the electrodes. In this way a display can be formed in a simple manner. The pixel structure is not limited to a particular shape. Basically all pixel shapes are possible leading to a segmented display, e.g. for showing icons or simple figures.
The light emitting polymers, oligomers, or low molecular weight compounds may be any electroluminescent material, such as polyfluorene copolymers such as disclosed in WO 97/33323, polyspiro copolymers such as disclosed in DE 19615128, poly(3-alkylthiophene), and poly(p-phenylene vinylene) (PPV) such as disclosed in WO 98/27136. Preferably, soluble conjugated polymers and oligomers are used because they can be easily applied, for example in a spin-coating process or by ink jetting. Preferred examples of soluble conjugated PPV derivatives are poly(dialkyl-p-phenylene vinylene) and poly(dialkoxy-p-phenylene vinylene). The light emitting material may also be a doped low molecular material, such as 8-hydroxyquinolin-aluminum doped with a dye, such as quinacridone, deposited in a vacuum process.
Dependent upon the preparation of the conjugated polymer, said polymer may comprise 5 to 10% non-conjugated units. It has been found that such non-conjugated units increase the electroluminescence efficiency, which is defined by the number of photons per injected electron in the active layer.
The above-mentioned conjugated PPV derivatives can be dissolved in the customary organic solvents, for example halogenated hydrocarbons such as chloroform, and aromatic hydrocarbons such as toluene. Acetone and tetrahydrofurane can also be used as solvents.
The degree of polymerization of the conjugated polymer ranges between 10 and 100,000.
The layer thickness of the light emitting layer of the conjugated polymer often ranges between 10 and 250 nm, in particular between 50 and 130 nm.
The LED structure can be provided on a substrate which is made, for example, from glass, quartz glass, ceramic, or synthetic resin material. Transistors or other electronic means may be present between the substrate and the transparent electrode forming a so called active matrix substrate. Preferably, use is made of a translucent or transparent substrate. If a flexible electroluminescent device is desired, use is made of a transparent foil of a synthetic resin. Suitable transparent and flexible synthetic resins are, for example, polyimide, polyethylene terephthalate, polycarbonate, polyethene, and polyvinyl chloride.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.